A "supergiant" gas field holding the equivalent of 5.5bn barrels of oil has been discovered off the coast of Egypt, the largest ever find in the Mediterranean.

Italian oil group Eni, which announced the discovery on Sunday, said the find was enough to supply Egypt with gas for decades, in a major boost to the country's struggling economy.

The deepwater Zohr well, almost 5,000 feet below the water's surface and covering 100 square kilometres (40 square miles), may hold 30 trillion cubic feet of gas or more. Eni said it would fast-track the development of the discovery to begin production as soon as possible.

"This historic discovery will be able to transform the energy scenario of Egypt in which we have been welcomed for over 60 years," said Claudio Descalzi, Eni's chief executive.

The Italian company is one of Europe's biggest oil groups and the biggest foreign energy company in Africa. Eni said Mr Descalzi had recently travelled to Egypt to discuss the findings with the country's president, Abdel Fattah Al-Sisi.

As well as being the biggest ever gas discovery in the Mediterranean, Eni said the Zohr field "could become one of the world’s largest natural-gas finds".

The discovery comes as major oil and gas discoveries become less frequent, with companies having to explore more remote areas in order to find promising wells.

"Our exploration strategy allows us to persist in the mature areas of countries which we have known for decades and has proved to be winning, reconfirming that Egypt has still great potential," Mr Descalzi said.

Egypt consumed 1.7 trillion cubic feet of natural gas last year, according to BP's most recent Statistical Review of World Energy. At the same rate of consumption, the Zohr discovery could supply the country for almost two decades.

Eni believes it can begin production as early as 2016. "Eni will immediately appraise the field with the aim of accelerating a fast track development of the discovery," said the company, which has a 100pc interest in the area it has discovered.